This Phase I SBIR proposal addresses the need to detect colon ischemia associated with the repair of Abdominal Aortic Aneurysm (AAA). AAA can be found in 5% of the adult population over 50. A leading complication of the repair of AAA is ischemia (about 5% rate of ischemia to the gut and/or lower limbs). Of the patients who develop moderate to severe colon ischemia during AAA repair, 57% die in hospital of irreversible colon infarction. Surgical intervention substantially improves the outcome of acute colonic ischemia, but only if the ischemia is detected and treated early. However, no FDA-approved early detection technique exists. We directly address this problem by seeking to detect colon ischemia early, prior to the onset of irreversible colon infarction. In this Phase I (R43) pilot trial we test the ability of a visible-light venous-oxygenation- sensitive rectal probe to detect acute colon ischemia during AAA repair. Such detection would ultimately allow for real-time vascular interventions already available to restore the colonic blood flow, reducing the lethality of acute colon ischemia. In this study, colon mucosal tissue hemoglobin saturation (StO2%) is measured with a rectal catheter that monitors hemoglobin in the small blood vessels (arterioles, capillaries, and venuoles) using shallow-penetrating, highly-localizing visible light (green to orange), allowing localized mucosal measurement. This approach differs from most optical methods by using shallow- penetrating visible light spectroscopy (VLS) that permits mucosal oxygenation monitoring via thin catheters. The specific aims of Phase I (R43) are (a) To monitor 20 patients undergoing AAA repair to establish normal StO2% values in patients with vascular disease, (b) to cross-validate VLS StO2% against laser Doppler measures of local blood flow, and (c) to modify the hemoglobin algorithm to include the pigmented urobilinogen breakdown products of heme in the stool to test if this reduces the influence of residual stool in the field and results in >2 % mean saturation difference. Later, when these milestones are met, we will submit a follow-on Phase II (R44) trial in which we test the catheter at 5 centers in a clinical test of catheter safety and efficacy in patients undergoing AAA repair. If successful, this grant will result in a manufacturable device to reduce the mortality of colon ischemia during AAA repair, by allowing for intervention at the onset of ischemia, prior to progression to colon death.